i18n: Rewrite gettext messages start with dash

Gettext message in a shell script should not start with '-', one
workaround is adding '--' between gettext and the message, like:

    gettext -- "--exec option ..."

But due to a bug in the xgettext extraction, xgettext can not
extract the actual message for this case. Rewriting the message
is a simpler and better solution.

Reported-by: Vincent van Ravesteijn <vfr@lyx.org>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -746,7 +746,7 @@ cmd_summary() {
 	if [ -n "$files" ]
 	then
 		test -n "$cached" &&
-		die "$(gettext -- "--cached cannot be used with --files")"
+		die "$(gettext "The --cached option cannot be used with the --files option")"
 		diff_cmd=diff-files
 		head=
 	fi